A noxious cloud of hydrogen sulphide engulfed portions of the Russian capital yesterday. The cloud, which can be highly toxic and smells like rotten eggs, may have originated from an oil refinery in the city.

There are conflicting reports as to the cause of the leak. As RT reports, the Emergencies Ministry is saying it came from a sulfur dioxide processing facility at an oil refinery in Moscow, and that the level of hydrogen sulfide — a gas that's toxic to humans — was over the permitted level for a short period of time while levels of hydrogen dioxide remained within the norm. The two gases are byproducts of oil processing.

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The refinery's owner, Gazprom Neft, said there was no accident and the levels of hydrogen sulphide at the plant were not excessive.

According to Vestnik Kavkaza, the Moscow Eco Monitoring systems of pollution control in the south east of Moscow registered an excess of six to seven times of hydrogen sulphide at 10:00 a.m. yesterday. The hydrogen sulphide exceeded the maximum permissible discharges 3-fold at 4:00 p.m. and returned to a permissible level at 6:00 p.m.

The spread of the cloud induced a wave of near-panic across the city. It was detected in the capital's main shopping areas and around the parliament buildings. The noxious gas, in addition to being foul smelling, is making life miserable for some residents, inducing headaches, dizziness, nausea, and irritation in the eyes.

Some Muscovites took to social media as the cloud rolled through.

The U.S. Bureau of Diplomatic Security put out the following bulletin to American citizens visiting the capital:

Russia's Ministry of Emergency Situations is advising residents of Moscow to remain indoors. Multiple sources are reporting an industrial accident in Kapotnaya, Russia (in the Southeast suburbs of Moscow) resulting in a leak of hydrogen sulfide, commonly associated with sewer gas. There is no official confirmation of this event at this time, nor is there confirmation whether the alleged gas leak involves toxic levels that could be harmful to health. However, U.S. citizens are strongly advised to remain indoors, to close windows and doors, and, if possible, to turn off heating and cooling systems to reduce the flow of outside air into closed environments. U.S. citizens who experience adverse symptoms, such as severe pulmonary distress, should seek assistance from local authorities. U.S. citizens are advised to monitor local news sources. The U.S. Embassy in Moscow will provide updated information as it becomes available.